Brain dopaminergic system changes in drug addiction: a review of positron emission tomography findings

Abstract

Dopamine (DA) is considered crucial for the rewarding effects of drugs of abuse, but its role in addiction remains unclear. Positron emission tomography (PET) is the fi rst technology used for in vivo measurement of components of the dopaminergic system in the human brain. In this article, we review the major fi ndings from PET imaging studies on the involvement of DA in drug addiction, including presynaptic DA synthesis, vesicular monoamine transporter 2, the DA transporter, and postsynaptic DA receptors. These results have corroborated the role of DA in addiction and increased the understanding of its underlying mechanisms.Keywords: dopamine; dopaminergic system; drug addiction; positron emission tomography ·Review· Brain Dopaminergic System Dopamine (DA) is a catecholamine neurotransmitter in the nervous system. It has many functions in the brain, including punishment, reward, voluntary movement, mood, attention, motivation, sleep, working memory, and learning [1] .DA is synthesized by the hydroxylation and decarboxylation of L-tyrosine in DA neurons and, before being released into the synapse in response to an action potential, it is stored within presynaptic vesicles (Fig. 1). The action of DA occurs by binding to postsynaptic DA receptors, resulting in the formation of second messengers. There are fi ve subtypes of DA receptors, which can be grouped into two classes or families: D1-like and D-2 like [1,2] . The D1-like receptor family comprises the D1 and D5 receptors, encoded by genes with no introns, acting by way of Gs-proteins and activating adenylyl cyclase, thus increasing cAMP production [3,4] .The D2-like receptor family comprises the D2, D3, and D4 receptors, encoded by genes containing introns. D2-like receptors act via Gi-proteins, inhibit adenylyl cyclase activity, and thus decrease cAMP activity [3,5] .The action of DA in the synapse is terminated primarily by reuptake to the presynaptic membrane through the dopamine transporter (DAT) [6] . Otherwise, DA is partially removed by oxidation by monoamine oxidase orcatechol-O-methyltransferase in the synaptic cleft (Fig. 1).T h e d o p a m i n e r g i c n e u r o n s , w h o s e p r i m a r y neurotransmitter is DA, interconnect many areas of the brain to form a system which originates in the substantia nigra (SN) pars compacta, ventral tegmental area (VTA), and hypothalamus. The dopaminergic system is typically divided into four major pathways: mesocortical (from the VTA to the frontal cortex), mesolimbic (from the VTA to the nucleus accumbens), nigrostriatal (from the SN to the striatum), and tuberoinfundibular (from the hypothalamus to the pituitary gland). The mesostriatal and mesocortical pathways are currently recognized to contribute most to drug addiction [7] . Neurosci Bull October 1, 2014, 30(5): 765-776 766 Drug AddictionThe term addiction is derived from the Latin verb addicere, which referred to the Roman court action of binding a person to another. From the 17th century, addiction has been used to refer to psychoactive substances (...

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